The DMSO currently available on the market is a product which is already of good purity. Its commercial specifications are generally:
purity .gtoreq.99.7% by chromatography acidity .ltoreq.0.04 mg KOH/g by potentiometry crystallization .ltoreq.18.1.degree. C. point visual appearance .ltoreq.clear water content .ltoreq.0.15% colour (APHA) .ltoreq.10
Patent FR 2 014 385 describes a process for the preparation of purified DMSO using an ion exchange resin. In both examples of this patent a strongly basic resin of the Amberlite IR-A 400 or Merck III type is employed to treat dimethyl sulphide/DMSO/10% sulphuric acid ternary mixtures. In fact, in this known process the purification seems to be essentially brought about by a fractional distillation of an aqueous solution of DMSO treated beforehand with an anion exchanger.
Analyses of trace metals have now been performed on a number of samples of commercial DMSO from various sources. These analyses are reported in Table 1.
The sodium, iron, potassium, calcium, chromium, copper, nickel and zinc concentrations were measured using ICP (plasma torch--atomic emission spectrometry, Perkin Elmer instrument, Optima 3000 model) and are expressed in ppb (1 ppb=1 part by weight per thousand million=1 .mu.g per kg).
The list of the metallic elements which appears in Table 1 is not exhaustive in respect of the metallic elements present in these samples.
TABLE 1 Metal cations Sample Na Fe K Ca Cr Cu Ni Zn 1 40 13 60 20 2 10 8 10 2 39 60 3 13 13 &lt;2 18 3 3 30 40 3 20 12 &lt;2 15 3 4 30 40 3 14 13 &lt;2 15 3 5 30 &lt;1 20 25 &lt;2 &lt;2 &lt;3 &lt;3 6 70 90 65 55 15 2 25 60 Detection 2 1 3 2 2 2 3 3 limit
For some applications, as for example in electronics or in pharmacy, the DMSOs analysed above contain too many metallic impurities. A DMSO containing less than 10 ppb of each alkali and alkaline-earth metal and metal contaminant would generally be necessary for most of the uses in the abovementioned two technical fields.